Know your itinerary. If you know about the dressy events, casual evenings and business days you have in store, you can plan accordingly and avoid bringing unnecessary outfits.
Research the weather at your destination. If summer days are notoriously dry, take a risk and skip the umbrella; if nighttime temperatures are known to plummet, pack layers for warmth.
Plan your clothes around one main coordinating color. Black and khaki are good, neutral 'foundation' colors that you can dress up, down, add color to and accessorize for different events and different days. Consider bringing dark colors and prints that don't show dirt.
Choose clothes for lightness and washability. Select several light layers that pack down easily and dry quickly, should you need to do hotel-room washing on the fly. Avoid bulky items like sweaters and heavy coats if you can be comfortable in two to three interchangeable layers.
Pack a couple of favorite scarves or belts for variety.
Pack a fanny pack or small backpack for day trips. If you're planning to shop, pack a lightweight duffel bag.
Wear your bulkiest outfit while traveling to save space in your luggage.
